Flat panel displays are used in a variety of applications ranging from relatively large devices including computer monitors and televisions, to small, handheld devices such as cell telephones, portable DVD players, wristwatches, and gaming devices. Many flat panel displays use optically active materials, such as liquid crystals, and a light source for backlighting the optically active materials. Films disposed between the liquid crystals and a backlight have been used to enhance the brightness of the displays. For example, brightness enhancement films may be used to increase the light exiting at a desired viewing angle with respect to the surface of the display. Light that emerges substantially parallel to the desired viewing angle is referred to as “on-axis” light. In many applications, the desired angle of the exiting light is perpendicular to the display surface. Increasing the amount of on-axis light reduces the amount of energy required to generate a desired amount of on-axis luminance. This is particularly important for optical displays that use battery powered light sources.
In general, the increase in on-axis brightness produced by a brightness enhancement film is known as the “gain” of the film. For example, if the desired viewing angle is perpendicular to the surface of the display, the on-axis gain of a film refers to the ratio of the intensity of light as measured substantially perpendicular to the surface of the display with and without the film.
Brightness enhancing films having one substantially flat surface and another surface having prismatic structures are frequently used to direct light that would otherwise not be viewed along the viewing axis. A typical flat panel display device may use several different types of films to provide an overall bright, high contrast display with substantially uniform output along the preferred viewing directions.
There is a need for enhanced backlights to provide lighting for optical displays, particularly for low power applications. The present invention fulfills these and other needs, and offers other advantages over the prior art.